The invention relates to a method of operating a power-assisted hydraulic steering system of a motor vehicle, having a hydraulic pump mechanically driven by the vehicle drive assembly, and having an additional function which reduces the amount of power assistance at a high driving speed in comparison to the amount of power assistance at a low driving speed and, in the process, simultaneously increases the amount of the hydraulic medium circulated by way of the hydraulic assistance unit of the steering mechanism.
For the technical background, in addition to European Patent Document EP 0 962 377 A2 and U.S. Pat. No. 6,073,721, the specification of which is expressly incorporated by reference herein, reference is also made particularly to the system description of the Servotronic® of the SF steering systems known to a person skilled in the art.
Hydraulic steering systems having the characteristics noted above are offered by various steering system manufacturers and are characterized in that, the so-called additional function, i.e., the so-called characteristic manual steering moment-pressure valve, curve of a power steering valve can be changed or is changed as a function of the driving speed of the motor vehicle. In the standing position or at a low driving speeds (for example, when parking), such a speed-dependent hydraulic steering makes it possible for the driver of the vehicle to comfortably steer through as a result of a high power assistance moment and, at higher driving speeds, offers a good acknowledgement because of a reduced or low assistance moment.
In the case of known hydraulic steering systems having the characteristics noted above, the pressure in the hydraulic system may rise considerably under certain operating conditions when only a small hydraulic quantity is guided by way of the so-called hydraulic power assistance unit of the steering mechanism—this is normally a piston cylinder unit which is connected in series, for example, with a toothed rack of the mechanical toothed-rack steering gear. In this case, the hydraulic pump of the hydraulic steering system is operated at an unchanged pumping capacity or at a pumping capacity which is a function of the rotational speed of the vehicle drive assembly driving this pump, so that quasi-excess pumping capacity is converted to thermal energy for the most part inside the pump, whereby the pump is subjected to extreme stress or, in extreme cases, may even be damaged.
The present invention provides remedies for the above-described problems.
One aspect of the invention provides a power-assisted hydraulic steering system having a hydraulic pump mechanically driven by the vehicle drive assembly, and having an additional function which reduces the amount of power assistance at a high driving speed in comparison to the amount of power assistance at a low driving speed and, in the process, simultaneously increases the amount of the hydraulic medium circulated by way of the hydraulic assistance unit of the steering mechanism. The above-mentioned additional function is activated independently of the driving speed also under operating conditions which could subject the hydraulic pump to extreme stress. Different operating conditions of this type are described and claimed herein.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.